Trona deposits in Sweetwater and adjacent counties in Wyoming are found at depths of about 1500 feet underground and consist of a main trona bed varying from 8 to 10 feet in thickness. The trona consist mainly of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O) containing about 3 to 15% or, more generally, 5 to 10% of insoluble impurities, mainly shale.
A typical natural trona composition is given below:
______________________________________ Na.sub.2 CO.sub.3 41.8% NaHCO.sub.3 33.1% H.sub.2 O 14.1% 89.0% Insolubles: Dolomite CaCO.sub.3 , MgCO.sub.3 5.5% Quartz SiO.sub.2 1.1% Feldspar: (K,Na).sub.2 O. Al.sub.2 O.sub.3 . ySiO.sub.2 . zH.sub.2 O 3.3% Clay: 2K.sub.2 O . 2MgO . 8Fe.sub.2 O.sub.3 24SiO.sub.2 . 12H.sub.2 O 0.6% Shortite: Na.sub.2 CO.sub.3 . 2CaCO.sub.3 0.1% Carbonaceous Matter, as Elemental Carbon 0.2% Other (by Difference) 0.2% ______________________________________
In the production of sodium carbonate from natural trona, it is usual procedure to calcine the trona at elevated temperature to obtain crude sodium carbonate: EQU 2(Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O) .fwdarw. 3Na.sub.2 CO.sub.3 + 5H.sub.2 O + CO.sub.2.
Calcination of the trona effects moisture removal, reduction or elimination of organic matter as well as transformation of the sodium sesquicarbonate to sodium carbonate. However, calcination at elevated temperature also causes sodium carbonate to react with silica contained in the trona to form soluble silicates. The quantity of soluble silicates so formed increases materially with increase in calcination temperature. Soluble silicates are a major impurity in solutions obtained by dissolving calcined trona and, unless steps are taken to reduce soluble silicate content of such solutions, soluble silicates become a significant impurity in soda ash prepared from trona.
Further, the trona contains carbonaceous matter which is at least partially soluble in the sodium carbonate process liquors. These carbonaceous impurities detrimentally affect the geometrical physical properties of the sodium carbonate obtained from these liquors. To avoid these difficulties, it is usual practice to contact the sodium carbonate process liquors with activated carbon to selectively absorb the carbonaceous matter therefrom. Still, not all of the critical carbonaceous habit modifiers respond favorably to the activated carbon treatment.
Levels of soluble silicate and carbonaceous impurities in sodium carbonate mother liquors derived from trona usually are controlled or maintained by purging part of the mother liquor. This, of course, entails loss of valuable soda values and poses a disposal problem.
It has already been proposed to reduce soluble silicate levels in aqueous solutions of sodium carbonate obtained by dissolving calcined trona in aqueous media by digesting the solutions at elevated temperature above about 160.degree. F. for time sufficient to substantially reduce the soluble silicates contained therein. Apparently, there is some interaction between the insoluble impurities and the soluble silicates in such solutions, rendering insoluble or absorbing the soluble silicates to reduce their concentration in the solution on digestion.
In our co-pending U.S. application Ser. No. 587,585 filed of even date herewith, there is described an improvement in the process of making sodium carbonate from trona by the method involving crushing the trona, calcining it and dissolving it in an aqueous medium for purification by crystallization, which improvement results in reduction of silicate contamination of the crystallization liquor and which improvement involves (a) segregating the calcined trona into a coarse and fine particle size fraction; (b) separately dissolving the coarse and fine particle size fractions to obtain aqueous solutions of sodium carbonate, sodium silicates and insoluble impurities; followed by (c) digesting the solution of the fine particle size fraction at elevated temperature to insolubilize soluble silicates, and separating the digested solution from insoluble impurities.
Further, in our co-pending U.S. application Ser. No. 587,575 filed of even date herewith there is described a method for reducing soluble silicate content of crystallizer mother liquor used in the process for making sodium carbonate from trona from which liquor sodium carbonate precursor crystals are obtained by evaporative crystallization, which method comprises (1) digesting the mother liquor at elevated temperature for time sufficient to reduce soluble silicates in the presence, as treating agent, of insolubles which have been obtained by calcination of crushed trona, segregation of a fine particle size fraction from the calcined trona, dissolution of said fine particle size fraction in an aqueous medium to obtain a solution comprising sodium carbonate and insolubles followed by separating insolubles and (2) separating the insolubles from the digested mother liquor, and recycling the mother liquor to the process.